1. Field of the Invention
The present invention relates to efficient amplification of radio frequency (RF) power, and more particularly, to a linear amplifier for ultra high frequency (UHF) applications which combines an inductive output tube with a multistage depressed collector to achieve substantially constant efficiency.
2. Description of Related Art
The advent of high definition television (HDTV) has provoked renewed interest in the efficient amplification of UHF signals. HDTV transmitting systems will require amplifiers capable of extremely high data rates on the order of twenty-five megabits per second. To support these high data rates, digital modulation techniques, such as four or six level vestigial sideband modulation or 16 or 32 state double sideband quadrature amplitude modulation (QAM) are proposed. These forms of modulation, when used in a channel of limited bandwidth (for example 6 MHz), result in signals which have high ratios of peak to average power. It is extremely difficult to amplify such signals both efficiently and faithfully, that is, with very low distortion of the modulation content as measured by the absence of high-order intermodulation products. Thus, RF linear amplifiers capable of providing these characteristics are very desirable.
Traditionally, klystrons were used as the high power amplifiers for most UHF transmitters. A klystron is a linear beam device having an electron beam which is passed through a plurality of cavities. An RF input signal velocity modulates the beam and causes it to become bunched. The bunched beam induces an RF current in the cavities, and energy can be extracted from the bunched beam as an amplified RF output signal. However, klystrons are very inefficient at output powers lower than the maximum for which they are designed since they operate at constant voltage and current, and their efficiency is proportional to the output power.
A known technique for increasing the efficiency of a klystron is the use of a multistage depressed collector (MSDC). The electrons of the velocity modulated beam have widely varying energy levels as they exit from the output cavity. By using a multiplicity of collector electrodes which are depressed to potentials below that of the device body (i.e. the potential corresponding to the original electron beam energy), the spent electrons of the beam can be collected at the minimum possible energy. The electrons may be considered analogous to balls having various velocities that might roll up a hill until they stop and then roll back into traps on either side of their upward path. By recovering most of the remaining kinetic energy of the spent electron beam in depressed stages, beam energy is not lost by conversion of the kinetic energy into heat, and higher operating efficiency can be achieved. Multistage depressed collectors are described in Kosmahl, Modern Multistage Depressed Collectors--A Review, Proceedings of the IEEE Volume 70, page 1325 (1982).
The efficiency of MSDC klystrons averaged over the modulation cycle has been shown to be up to three times that of conventional klystrons. Since the voltage at which the electrons are collected is roughly proportional to the RF output voltage of the klystron and the beam current is constant, the efficiency of the MSDC klystron is proportional to the square root of the output power. Despite this improved efficiency, MSDC klystrons do not provide the linearity necessary for the proposed HDTV transmitting systems.
Another type of amplifier utilizes one or more grids disposed between a cathode and an anode to density modulate current drawn from the cathode. It is a common practice to differentiate between amplifiers which use a grid to density modulate the electron stream on the basis of their operating regime, and they are categorized as either Class A, B or C. In a Class A amplifier, the grid bias and alternating grid voltages are applied such that the cathode current flows continuously through the electrical cycle. In a Class B amplifier, the control grid is operated at close to cutoff such that cathode current flows only during approximately half of the electrical cycle. Class AB amplifiers are hybrids of Class A and Class B amplifiers in which grid bias and alternating grid voltages are such that the beam current flow appreciatively more than half but less than the entire electrical cycle. Class C amplifiers have the grid bias appreciably greater than cutoff so that cathode current flows for appreciably less than half of the electrical cycle.
At lower frequencies, Class B amplifiers using triodes or tetrodes have demonstrated an ability to produce power more efficiently than conventional klystrons. In these amplifiers, the RF output current varies linearly with the cathode current and the voltage is constant, so the efficiency again varies as the square root of the output power as it does in the MSDC klystron. Tetrode and triode Class B amplifiers are effective for very high frequency (VHF) operation.
The advantages of Class B operation can be extended to higher frequencies by using a device known as an inductive output tube. Inductive output tubes have the same efficiency as other Class B amplifiers due to the fact that the RF input signal applied to a control grid causes the electron beam current to vary roughly as the RF drive voltage. Since the RF current in the tube does not result from velocity modulation, the amplifier is additionally highly linear.
The original inductive output tube was developed by A. V. Haeff, and consisted of a tubular glass envelope containing a cathode, a control grid disposed in front of the cathode, an accelerating aperture electrode and a collecting electrode. A gap of a reentrant cavity was disposed in part of the tubular glass envelope between the accelerating aperture electrode and the collecting electrode. The electron beam generated by the cathode passed through the gap when focused by a magnetic field. When the electron beam was density modulated by the application of an RF input signal to the control grid at a frequency equal to the resonant frequency of the cavity, the electron beam current induced an electromagnetic wave in the cavity which extracted energy from the electrons without intercepting the electrons. The inductive output tube had the advantage over earlier vacuum tubes in that the interaction gap of the cavity could be of small area and have a low capacitance suitable for high frequency operation, while the electrons could be collected on a much larger collector electrode which no longer needed to be part of the resonant circuit.
The original concept for the inductive output tube was later recognized as being advantageous for use as a linear amplifier for UHF television signals. A modernized inductive output tube is disclosed in U.S. Pat. No. 4,480,210 for GRIDDED ELECTRON POWER TUBE, which includes a highly convergent electron gun with a pyrolytic-graphite control grid and a large collector. Making the control grid of pyrolytic-graphite, a highly refractory material, permits a much higher current density than previously possible in the original Haeff inductive output tube. This updated tube became known as a "klystrode" since it combined features of conventional klystrons with those of tetrodes; the klystrode has the resonant output cavity of a klystron, and the four electrode configuration of the tetrode.
Despite widespread knowledge of MSDC klystron and IOT efficiency enhancing techniques, a combination of the benefits of the inductive output tube with the multistage depressed collector was not actively pursued. The common wisdom in the art was that any improvement in efficiency gained by combining these features would be only on the order of 10% to 15% at peak power levels, and thus would not be worth the additional investment to modify existing designs. See Gilmour, Microwave Tubes, pages 196-200 (Artech House 1986). Moreover, it was believed that collector depression would require an increase in the cathode to anode voltage for a given power output, and if too much depression was used in attempting to increase efficiency, a deterioration in picture quality due to returned electrons across the cavity gap would result. See Preist & Shrader, The Klystrode--An Unusual Transmitting Tube With Potential For UHF-TV, Proceedings of the IEEE, volume 70, pages 1318 (1982).
Thus, it would be desirable to provide an RF amplifier for UHF applications that provides higher operating efficiency levels and linearity than that previously achieved in the art. Ideally, the RF amplifier would provide constant high efficiency at any power level rather than the square root of output power relationship provided by inductive output tubes and multistage depressed collector klystrons.